The present invention concerns improvements in catalyst systems for the control of exhaust gas emissions from vehicles.
The emissions from vehicle exhausts are now well known to cause pollution, health problems and ecological damage. For this reason, various governmental or quasi-governmental bodies have issued regulations giving maximum levels of the main pollutants, namely carbon monoxide (CO), unburnt hydrocarbons (HC), NOx and, in the case of diesel engines, particulates. Increasing standards have been published, to come into effect at various times in the future, with the eventual aim for example in the USA to meet demanding regulations known as xe2x80x9cULEVxe2x80x9d (Ultra Low Emission Vehicle), with a proportion of vehicles having zero regulated emissions xe2x80x9cZEVxe2x80x9d. A variety of strategies have been suggested, including a system described in WO 96/39576 (Johnson Matthey) which has been found in tests in cars to exceed the ULEV standards. This system principally uses a very low light-off catalyst, which starts conversion of CO and/or H2 immediately upon start-up of the engine, thus creating an exotherm which rapidly raises catalyst temperature to the point of HC light-off. (xe2x80x9cLight-offxe2x80x9d is understood to be the temperature at which a given amount, for example 50% by weight, of a reactant is converted.)
There remains, however, the need for a system that is robust and able to cope with the wide variety of gasoline grades marketed, especially with regard to high sulphur levels and other possible catalyst poisons, especially traces of lead. The above-mentioned WO 96/39576 states that it is preferred to use CO as a xe2x80x9cfuelxe2x80x9d for initiating HC light-off. It is acknowledged that most engines do not produce significant quantities of hydrogen in the exhaust, and therefore a secondary source of hydrogen, such as an on-board reformer, would be necessary if hydrogen were to be required to play a major part in speeding HC light-off. A reformer itself requires an appreciable time to start-up to produce hydrogen. Another feature of the described development is that it does not require so-called xe2x80x9cstarterxe2x80x9d catalysts mounted in the xe2x80x9cclose-coupledxe2x80x9d position very close to, or even within, the exhaust gas manifold. WO 93/18346 (South West Research) discloses the use of a first combustion chamber in an internal combustion engine to produce an exhaust gas which is treated by a water gas shift reactor to produce a gas enriched in hydrogen, and thereafter recycling that hydrogen-enriched gas to another combustion chamber, to reduce the overall emissions of unburnt hydrocarbons and nitrogen oxides. All additional hydrogen produced is consumed within the engine.
GB 2,277,045 (Ford Motor Co.) concerns an adoption of the xe2x80x9cEGIxe2x80x9d (Exhaust Gas Ingnition) system which utilises an afterburner, and requires the temporary trapping of unburnt hydrocarbons so that only CO and hydrogen reach an afterburner where the gases are ignited by a spark plug. It is to be noted that this disclosure requires the use of an afterburner separate from a catalytic convertor downstream of the afterburner. This disclosure does not contemplate either increasing the hydrogen content of the exhaust gases or utilising hydrogen except in an EGI system.